verification

Umm you cannot pay your bill here. This is a public forum of other customers.

You call AT&T's publicly listed phone number to pay your bill. There is no sending a code to your phone to do so. Alternatively you could log onto your account on the AT&T website.

You can use AT&T *code to make payments with your bank account, credit/debit card. On your phone go to Settings > Carrier Services that list services you can perform with your phone.  Also you can go to an AT&T corp store to make payments. Link below has links to AT&T *code services. 

https://www.att.com/support/article/wireless/KM1051617/

That would be a code to make a payment, not quite verification that the OP is asking.

I am curious why they thought they pay their bill in a forum with a code. Sounds strange.